The present invention relates to the field of magnetic recording, and more particularly pertains to improvements in impulse magnetic recording, providing write equalization of both phase and amplitude and combining pilot and binary data transition signals.
Impulse Magnetic recording is a process of recording signals on a magnetic medium, which is described in my copending application Ser. No. 898,217, filed Aug. 20, 1986, which is a continuation of my application Ser. No. 669,357, filed Nov. 8, 1984, now U.S. Pat. No. 4,625,245, which is a continuation-in-part of my application Ser. No. 560,103, filed Dec. 12, 1983, now abandoned. The entire contents of said patent and applications is accordingly incorporated herein by reference.
Briefly describing the process of impulse magnetic recording, it utilizes a magnetic recording medium and a magnetic record head, wherein one is moved relative to the other to define a path of traverse of the head along the medium, in any conventional form and configuration. In conventional magnetic recording, an electrical waveform is applied to the record head continuously during the entire time of the waveform. In the practice of impulse recording, however, only very short duration electrical impulses are applied to the record head. Each such impulse is representative of the instantaneous value of the electrical signal being recorded. By "very short duration", it is meant that the time duration of the impulse is only a small fraction of the time it takes for a point on the magnetic recording medium to effect a relative traverse across the effective recording gap of the record head. The effective recording gap, generally speaking, is not much greater than the physical record head gap, and in normal practice would not likely exceed twice the physical head gap. In practice, this small fraction can be in the range of 0.1, although it certainly can be smaller or larger in accordance with the teachings of the aforesaid applications. In addition to utilizing these very short recording impulses, in one form of impulse recording as may be applied in certain specific forms of impulse recording utilized in the description of the present invention, successive recording impulses are applied to the record head in a sequence of equal time intervals approximately equal to the time of relative traverse of a point on the recording medium across the effective recording gap of the record head. As explained in detail in said prior applications, each impulse, no matter how short in duration, if resolvable by the record head/magnetic medium interface, causes a recorded increment along the magnetic medium equal to the entire length of the effective recording field of the head, plus the length of traverse for the time duration of the impulse. If successive impulses are applied at the aforesaid time intervals, the successive recorded increments in the medium are essentially contiguous, providing a recorded continuum corresponding to the electrical waveform continuum being recorded. This recording process of successive recording impulses, whether the time intervals between impulses are equal, or not, is referred to as sequential impulse recording.
Magnetic impulse recording and the improvements therein that are the subject of the present invention, are generally applicable to various forms of magnetic recording, but are described herein and in the aforesaid application for purpose of illustration in relation to digital data recording, and specifically the NRZ form of representation of the data. It is understood, however, that the invention is not limited thereto, as will be apparent to those skilled in the art. Also, although the invention is described at times in terms most applicable to fixed, ring head, longitudinal recording on magnetic tape, it certainly is not limited to such embodiments.
One of the improvements in sequential impulse magnetic recording of the present invention, pertains to amplitude equalization of the recorded data, particularly in the recording of relatively long wavelengths. In one ideal mode of practice of sequential impulse recording of digital data, for maximum recorded density, a data bit cell would have a time duration approximately equal to the aforesaid time interval between recording impulses. When the data waveform contains a longer interval than a bit cell between data transitions, the recording of the long interval is obtained by successive impulses corresponding to the number of bit cells encompassed by that long interval. Some benefits which can be obtained from sequential impulse recording are limited by interaction field which preclude constant depth, surface recording for all wavelengths. The depth of recording in the magnetic medium will increase with each successive impulse recorded increment of a long interval between data transitions. Greater benefits can be derived from amplitude equalized, sequential impulse recording. In general, this amplitude equalized recording is obtained by appropriately reducing the amplitude of those recording impulses between data transitions (sustaining impulses). In doing so, of course, one loses the ability to overwrite previously recorded data, because the reduced amplitude signal would not accomplish saturation to overwrite the previous recording. In accordance with the present invention, not only is recorded amplitude equalization obtained, but also, overwrite is made possible by using an overwrite impulse in proper relation to a sustaining impulse, to obtain the desired amplitude equalized recording. It is also found advantageous to adjust the sustaining impulse clock phase (of any frequency) with respect to the data transition impulse clock to achieve improved phase equalization of the recording.
It is further found advantageous at times to utilize a separate clock of greater frequency than the bit cell clock, for generating the sustaining amplitude equalized impulses between transitions, in order to reduce the clock signal amplitude in the reproduced signal. Further, in accordance with the present invention, the sustaining impulse timing signal could be either a clock-rate or clock-rate related pilot signal, and for amplitude equalization purposes it would be necessary only to use burst pilot signals during the periods of long intervals between data signal transitions.
However, in accordance with the present invention, if one generates a pilot signal that is always phase coherent with all transitions of the data signal, and if one records that pilot signal with the data transition recording impulses as well as utilizing it to obtain amplitude equalized sustaining impulses for long interval data signals, one obtains further significant improvements in the record/reproduce process. It is found that by this means one obtains very significant reduction of timing errors for data transitions in the reproduced waveform, and in addition, a very important reduction in the width of pulses representative of data transitions in the reproduced waveform. Moreover, separate recovery of the recorded pilot signal affords an independent means of reconstructing the data clock in the absence of data transitions. As will be explained subsequently, a pilot frequency of 1.5 times the bit cell clock frequency is often well suited to the purposes of the present invention.
It is therefore one object of the present invention to provide for impulse magnetic recording of electrical waveforms.
Another object of the present invention is to provide for enhanced sequential impulse magnetic recording of digital data.
Another object of the present invention is to provide for enhanced sequential impulse magnetic recording of digital data to achieve record (write) equalization in phase and/or amplitude.
Still another object of the present invention is to provide for write equalized sequential impulse magnetic recording of digital data, with overwrite capability.
A further object of the present invention is to provide for pilot enhanced impulse magnetic recording of electrical waveforms.
Still another object of the present invention is to provide for pilot enhanced impulse magnetic recording of digital data electrical waveforms, wherein the pilot signal is always phase coherent with the data waveform transitions.
Still another object of the present invention is to provide for pilot enhanced, write equalized, sequential impulse magnetic recording of digital data electrical waveforms.
And still another object of the present invention is to provide for pilot aided clock recovery during reproduction of long intervals between recorded digital data transitions.